Markup languages and their Characteristics
SGML, or Standard Generalized Markup Language, is the original markup language used for defining formats in text documents. Formatting codes are defined in a separate file called a Document Type Definition file.
HTML, or Hypertext Markup Language, is the language used to instruct the browser how it should display a page. The instructions are referred to as tags.
DHTML, or Dynamic HTML, is used to control the appearance and behavior of Web sites. Using a combination of HTML, style sheets, and scripting language, DHTML produces animated HTML.
XML, or Extensible Markup Language, is a subset of SGML and was developed specifically for Web applications. XML tags that define data are created within the document. This enables the exchange of data between applications.
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XML 1.0 and Namespaces
XML 1.0 and Namespaces in XML provide a tag-based syntax for structuring data and applying markups to documents. Documents that conform to XML 1.0 and Namespaces in XML specifications may be made up of a variety of syntactic constructs such as elements, namespace declarations, attributes, processing instructions, comments, and text. This chapter provides a description of each of the structural elements in XML along with their syntax
Elements typically make up the majority of the content of an XML document.
Every XML document has exactly one top-level element, known as the document element.
Elements have a name and may also have children. These children may themselves be elements or may be processing instructions, comments, CDATA sections, or characters. The children of an element are ordered. Elements may also be annotated with attributes. The attributes of an element are unordered. An element may also have namespace declarations associated with it. The namespace declarations of an element are unordered. Elements are serialized as a pair of tags: an open tag and a close tag. The syntax
for an open tag is the less-than character (< ) immediately followed by the name of the element, also known as the tagname , followed by the greater-than character
(>). The syntax for a close tag is the character sequence <
immediately followed by the tagname, followed by the greater-than character. The children of an element are serialized between the open and close tags of their parent. In cases when an element has no children, the element is said to be empty.
A shorthand syntax may be used for empty elements consisting of the less-than character immediately followed by the tagname, followed by the character sequence